Many processes that occur in the human body operate according to a regular cycle of approximately 24 hours known as the circadian rhythm or circadian cycle. In particular, a person's sleep-wake cycle typically occurs over a 24 hour period during which their sleep requirement gradually increases until the person sleeps and reduces their sleep requirement back to a base level. Similarly, a person's core body temperature varies in a cycle over the same approximate 24 hour period.
Although the processes naturally occur over an approximately constant 24 hour period, a person's circadian rhythm can be influenced by external factors, that may extend the cycle, e.g., by keeping the person awake longer than normal, or shorten the cycle, e.g., by increasing the person's sleep requirement. Alternatively, the external factors may change the rate that a person progresses through their circadian cycle or change a person's position on their circadian cycle making them more awake or more ready for sleep than they would normally be at that time of day or point in their circadian rhythm. Changes to the person's circadian cycle or the position of a person on their circadian rhythm may affect each of the natural processes that occur in time with the circadian rhythm.